Lions and Tigers and ... White Squirrels?
Just what is a white squirrel, anyway? Is it a distinct species? Is it a mutation? These are some of my most FAQs. And the answer is ....... it depends. There is, in fact, one tree squirrel for which a white coat seems to be a characteristic of the entire species. Its an Oriental Tree Squirrel with the scientific name Callosciurus finlaysoni (Callosciurus translates as "beautiful squirrel") found in Thailand and other parts of South East Asia. So if you sighted a white squirrel here in North America outside of captivity, its almost certainly a color variant of one of our many native species of the genus Sciurus; in my neighborhood, that would be the Eastern Gray Squirrel (Sciurus carolinensis). Most of what follows will be in reference to that species but white coated Fox Squirrels and Red Squirrels have been sighted, as well.
There is much variation in squirrel coat color both locally and regionally. The general pattern of brown/gray on top and white below is considered the wild type from which other variations arose. These wild types, like most mammals, have dark eyes. The brown, gray, or even black color comes from the production of a pigment called melanin. Variation in coat color can arrise from change in either the genes that control the production of melanin itself, or of genes that control the pattern of melanin production. After all, most wild type gray squirrels have a white abdomen not because there are mutated genes in those cells but because there are regulatory genes which suppress the activity of melanin genes in these locations but not others. The white abdomen is adaptive. It makes the squirrel less visible from below against a light sky. Yet some squirrels have tan or ochre bellies. Black or melanistic squirrels produce excessive amounts of melanin in comparison to the wild type and in expanded regions of the body. These variants are often found at higher latitudes and the dark color is thought to be involved in thermoregulation. The point I am making here is that while all variation from wild type comes about by genetic change known as mutation, the word mutation carries a negative connotation, implying a freakish condition. In this case it produces a wealth of variation that may actually be appropriate for local conditions (including human preference). Thus, I prefer the term variant over mutant although I will use both below for variety.
With that said, white squirrels are just another color variant of this very variable species. The most common sightings of white squirrels are of isolated individuals with a completely white coat but dark eyes. This variant appears to spring up sporadically all over the species' range and then dies out again, only to pop up again somewhere else. I think these are spontaneous mutants of some gene that delegates the use of the pigment (melanin) gene, not mutants of the melanin gene itself. Instead of being produced in both skin cells and the eyes, it is only produced in the eyes. In another mutation, one in the genes that are directly responsible for producing melanin, no cells make the pigment and the squirrel is not only white but has pink or blue eyes. These true ďalbinosĒ are reported even less commonly, probably because without the eye pigment to reduce glare, their vision is impaired and they suffer more from falls. There are three known colonies of true albino squirrels including the best studied one in Olney IL where they make up approximately 20% of the population.
Still rarer seems to be the type of coat pattern we have here in Brevard NC. The coat is mostly white but there is a distinctive head patch and dorsal stripe. Although there is much variation in the amount of pigmentation, these white squirrels can produce melanin, not just in the eyes but in skin cells as well. Iím guessing but I think the gene responsible is probably what we call a regulator gene, effecting the distribution of hair color, not color itself. The region of white hair, normally restricted to the abdomen in a gray squirrel, is expanded at the expense of pigmented regions. This variant makes up ~25% of our gray squirrels, amounting to almost a thousand white variants in the city limits (3 square miles). Although it has been very successful in our area, it is rare elsewhere and most known sightings have a Brevard connection in one way or another (the Brevard squirrel was itself imported from Florida).
Occasionally, the frequency of white variants in an area increases sufficiently and remains stable long enough to establish what is referred to as a "colony." Dr. John Stencel of Olney Illinois restricts the use of "colony" to those locations in which at least 20 (in absolute numbers) individuals are regularly observed to be white in what is otherwise a normal gray population. I use a more "liberal" definition of a colony including all those communities in which white variants are repeatedly or regularly observed without any attempt to set absolute threshold numbers since those populations are probably harboring a permanent reservoir of the white-predisposing gene(s). But whatever the definition, there are to my knowledge, no colonies of the Eastern Gray Squirrel approaching 100% white in North America. I believe that Brevard's population, with approximately one in four white, has the highest percent white of any known colony and with almost a thousand individuals bearing the white coat color in any given year, it is far and away the largest such "colony."